Clays are composed of infinite layers (lamellae) of metal oxides and hydroxides stacked one on top of the other. These layers or sheets are composed of tetrahedrally coordinated cations which are linked through shared oxygens to sheets of cations octahedrally coordinated to oxygens and hydroxyls. When one octahedral sheet is linked to one tetrahedral sheet a 1:1 layered structure is formed as in kaolinite, whereas when one octahedral sheet is linked to two tetrahedral sheets, a 2:1 layered structure is produced as in beidellite. Anionic charges on the tetrahedral layers (usually siliceous layers) are neutralized by cations such as Na.sup.+ or Ca.sup.+2 in the interlamellar spaces. These cations can be exchanged with other cations.
Some of the clays are swellable, i.e., they swell or expand when placed in water or other solvents. These clays include the smectite group of clays which are 2:1 layered clays. Included in the smectite group are montmorillonite, beidellite, nontronite, hectorite, saponite, Laponite.TM., and sauconite. The clay layers in these swellable clays can be propped open or pillared with large cations such as Fe.sup.+3, Cr.sup.+3 or with metal hydroxy polymer cations such as (Al.sub.13 O.sub.4 (OH).sub.24 (H.sub.2 O).sub.12).sup.+7 or (Zr(OH).sub.2.4H.sub.2 O.sub.4.sup.+8.
Pillared clays are known to catalyze numerous reactions such as alkylation, cracking, ester formation, dimerization, oligomerization, etc. A review of the reactions catalyzed by pillared clays may be found in an article by J. M. Adams, Applied Clay Science, 2, pp. 309-342 (1987). Of these reactions, alkylation has received considerable attention. For example, U.S. Pat. No. 4,499,319 discloses layered clays such as montmorillonite which have been ion-exchanged with metal cations such as chromium and aluminum, which are used to alkylate aromatic compounds. Other examples include U.S. Pat. No. 4,605,806 which discloses a hydrogen ion-exchanged pillared clay; U.S. Pat. No. 3,965,043 discloses a metallic cation exchanged trioctahedral 2:1 layer-lattice smectite-type clay and U.S. Pat. No. 3,979,331 which discloses a metallic cation exchanged synthetic hectorite-type clay useful for alkylating aromatic hydrocarbons.
It is also reported that the swellable clays can be fluorided and then pillared. For example, Butruille et al. in J. of Catalysis, 139, 664-678 (1993) disclose synthesizing a fluorided hectorite clay which is then pillared with an Al.sub.13 polycation and subsequently calcined. The authors report enhanced catalytic activity for propylene alkylation relative to nonfluorinated smectite hosts.
In contrast to this art, applicant has prepared a pillared smectite clay in which the pillars have been fluorided. For example, a montmorillonite clay was pillared with aluminum chlorohydrate (ACH), calcined and then treated with a fluoride salt such as ammonium fluorosilicate. Activity testing shows that the fluorided pillared clay of this invention has enhanced activity versus a fluorided clay that has been pillared.